Some coal boiler furnaces in use today were designed two to three decades ago. For economical reasons, these older furnaces were designed with an undersized firebox. Consequently, the furnace lacks enough surface area to effectively absorb the heat generated by the burning of the coal, and temperatures in the furnace are elevated. When the temperature of the flue gas at the exit of the firebox exceeds the specific coal ash melting temperature, the ash in the flue gas melts or partially melts so that ash deposits on the surfaces of the furnace, resulting in hard slag deposits.
These older coal boilers have been converted in the intervening years to burn types of coal that are different from what the boiler furnaces were originally designed for. Younger coal deposits are being used presently, and since younger coal tends to have more volatile content, the temperatures reached in the furnace are higher. Similar to and in conjunction with the problems related to an undersized furnace, the temperature of the heated flue gas at the exit of the firebox exceeds the ash fusion temperature, resulting in slagging and fouling of the surfaces of the waterwalls, firebox, superheater, and reheater of the steam boiler.
The problem of achieving a reduction in the temperature of the flue gas at the exit of the firebox to just below the specific ash melting temperature in order to control the slagging and fouling of the steam boiler has not been adequately resolved.